The use of mobile phones and wireless broadband devices has increased rapidly during the last decade, and it is expected to grow even faster during coming years. To meet these demands, operators must increase the capacity in their communication systems.
A well-known way of increasing the capacity in communication systems is to integrate more than one antenna and radio chain, both at the base stations and at the user terminals, and use so-called MIMO (Multiple Input Multiple Output).
Today, a user terminal such as a mobile phone is mostly used for voice communication, but a clear trend is that more and more people are using their mobile phones for data applications, e.g. streaming movies and music, chat, and the Internet in other ways. One problem with mobile phones is that their battery capacity is relatively limited, and it is therefore needed to re-charge the mobile phone battery frequently. In the future when more radio chains will be integrated in the terminals, the power consumption will increase and the battery must be recharged even more frequently. There is therefore a need for reducing the power consumption to increase the time between charging. In addition the carbon-dioxide “footprint” of the device will be smaller.
WO 2009/080110 describes a device that combines two antenna elements using a beam-forming network and thereby optimizing the antenna gain and polarization to the environment. However, the number of antennas and the number of active radios are fixed.
A mobile phone, or another wireless broadband device constituting a user terminal, in a MIMO system may experience a rank-one channel or have no need for high data rates. For example, in line-of-sight scenarios between a user terminal and a base station, the propagation channel often only supports one stream, unless both ends of the link have dual polarized antennas. In such cases, the user terminal cannot, or does not, need to utilize multiple streams.
A similar situation is present in other types of nodes such as base stations and repeater stations.
There is thus a need for a more efficient use of antennas and radio chains in a node.